1. Field of the Invention
The present invention relates to a heat fixing unit in an electrophotographic copying apparatus.
2. Description of the Prior Art
The conventional heat fixing unit in an electrophotographic copying apparatus has a heat fixing roller 30a and a pressure roller 40a as shown in FIG. 16.
The heat fixing roller 30a and the pressure roller 40a nip both sides of a copying paper 20a with toner images 10a formed on one of its surfaces. The heat fixing roller 30a and the pressure roller 40a deliver the copying paper, heat and supply pressure to the toner images 10a in order to fix the toner images 10a on the copying paper 20a.
3. Problems with the Prior Art
The conventional heat fixing unit fixes the toner images 10a on the copying paper 20a by pressing the copying paper 20a and the toner images 10a hard against the heat fixing roller 30a with the pressure roller 40a contacting with the reverse surface of the copying paper 20a, which is opposite to the toner images 10a formed surface, and by heating the copying paper 20a and the toner images 10a with the heat fixing roller 30a. The pressure roller 40a employs a material having a larger elasticity than that of the heat fixing roller 30a in order to increase the fixing time by widening the nip width of the copying paper 20a in relation to the heat fixing roller 30a. For instance, the pressure roller 40a employs a rubber roller. Consequently, the conventional heat fixing unit in an electrophotographic copying apparatus has the following problems:
It is necessary to increase the strength of retaining plates for retaining the pressure roller 40a because the pressure roller 40a is brought into contact with the heat fixing roller 30a by the large force of approximately 1.5 kgf/cm. (Pressure, force or pressing force means the force per axial length of a roller in this specification. The total pressing force means the product of the pressing force and the axial length of a roller.) It is a also necessary to increase the strength of the heat fixing roller 30a and the pressure roller 40a themselves.
As the area of the pressure roller 40a contacting with the heat fixing roller 30a is large in the conventional heat fixing unit, a large quantity of heat escapes from the heat fixing roller 30a to the pressure roller 40a. Accordingly, it is required to prolong the warming up time for the heat fixing unit in the initial stage of the electrophotographic copying apparatus operation, and it is also required to heat and keep the heat fixing roller 30a at a high temperature. As a result, the quality of the copying paper 20a is liable to change during the delivery between the heat fixing roller 30a and the pressure roller 40a; i.e. many wrinkles are liable to occur because a large quantity of water is removed from the copying paper 20a. Hence, the copying paper 20a tends to curl.
When fixing at high pressure, the bonding force between the heat fixing roller 30a and the toners forming the toner images 10a increases. Consequently, it is hard to separate the copying paper 20a from the heat fixing roller 30a.